This invention pertains to a method of preparing a fumed metal oxide dispersion that is useful in a variety of applications.
There are many applications for fumed metal oxides, particularly for fumed silica, of extremely fine particle size in which it is convenient to apply the fumed silica in the form of a dispersion. Such applications include foamed rubber lattices, paper coatings (i.e., recording media), the manufacture of optical fibers and quartz glassware, thermal insulation, and chemical-mechanical polishing (CMP). There also are many occasions where it is convenient to densify fumed silica for storage or transport by combining the fumed silica with water to form an aqueous dispersion.
Fumed silica is generally produced by the vapor phase hydrolysis of chlorosilanes, such as silicon tetrachloride, in a hydrogen oxygen flame. The overall reaction is:
SiCl4+2H2+O2xe2x86x92SiO2+4HCl 
In this process, submicron sized molten spheres of silica are formed. These particles collide and fuse to form three dimensional, branched, chain-like aggregates, of approximately 0.1-0.5 xcexcm in length. Cooling takes place very quickly, limiting the particle growth and insuring the fumed silica is amorphous. These aggregates in turn form agglomerates of 0.5-44 xcexcm (about 325 US mesh). Fumed silicas generally have very high purity, with total impurities, in many cases below 100 ppm (parts per million). This high purity makes fumed silica dispersions particularly advantageous for many applications.
In order to be useful for the applications listed above and other potential applications, the fumed metal oxide dispersion cannot coagulate. The ability of the dispersion to resist gelling is generally referred to as the stability of the dispersion. In general, the more stable the dispersion, the slower the dispersion will coagulate. Dispersions tend to coagulate when the solid loading is too high or when the dispersion reaches a certain pH level. Therefore, it would be desirable to produce a dispersion that could be prepared with both a high solids loading and at a high pH while still remaining stable for an extended period of time.
Attempts to make stable dispersions have involved a number of different methods. U.S. Pat. No. 5,246,624 discloses a method of preparing an aqueous colloidal dispersion comprising fumed silica, an acid, and a stabilizer. Similarly, U.S. Pat. No. 2,984,629 discloses a method of preparing aqueous dispersions of metal oxides comprising mixing the metal oxide with water and subjecting this mixture to high shear in the presence of a sufficient amount of an alkali hydroxide and a dispersing agent.
While these methods are known, there still remains a need for improved methods of preparing stable dispersions of fumed metal oxides, particularly with high solids loadings and at relatively high pH. The invention provides such an improved method. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The invention provides a method of preparing a fumed metal oxide dispersion. The method comprises providing a liquid carrier at a pH that effects dissolution of the metal oxide at a rate the same as or greater than the metal oxide dissolution rate in water at a pH of about 8. One or more aliquots of both a fumed metal oxide and a metal ion source then are mixed with the liquid carrier, in any order, to form a dispersion, such that the dispersion does not coagulate. The pH of the dispersion can be optionally adjusted to the initial pH of the liquid carrier.